1. Field of the Invention
The present invention relates to ink compositions for use in ink jet printers. More particularly, it relates to the use of carboxylated lignin in water-based, ink jet inks. Most particularly, it relates to ink compositions with excellent water resistance properties while achieving excellent print quality, jetting properties, storage stability, reliability, and drying times.
2. Description of the Related Art (Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98)
Ink jet printing involves placement, in response to a digital signal, of small drops of a fluid ink onto a surface to form an image without physical contact between the printing device and the surface. The method of drop generation varies among the different ink jet technologies and can be used to classify ink jet printing into two major technology types, continuous (CIJ) and drop-on-demand (DOD).
In DOD printing systems, liquid ink droplets are propelled from a nozzle by heat (thermal or bubble ink jet) or by a pressure wave (piezo ink jet). All the ink droplets are used to form the printed image and are ejected as needed, "on demand." Thermal or bubble jet inks typically are based on water and glycols. Piezo ink jet systems generally use aqueous, solvent, or solid inks. These last inks, also known as phase change inks, are solid at ambient temperature and are liquid at printing temperatures.
In CIJ printing systems, a continuous stream of liquid ink droplets is ejected from a nozzle and is directed, with the assistance of an electrostatic charging device in close proximity to the print head, either to a substrate to form a printed image or to a recirculating system.
The following properties are required of an ink composition for ink jet printing:
(a) high quality printing (edge acuity and optical density) of text and graphics on substrates, PA1 (b) short dry time of the ink on a substrate such that the resulting printed image is not smudged when rubbed or offset onto a subsequent printed image placed upon the print, PA1 (c) good jetting properties exhibited by a lack of deviation of ink droplets from the flight path (misplaced dots) and of ink starvation during conditions of high ink demand (missing dots), PA1 (d) resistance of the ink after drying on a substrate to water and to abrasion, PA1 (e) long-term storage stability (no crust formation or pigment settling) and PA1 (f) long-term reliability (no corrosion or nozzle clogging). PA1 (a) from about 40% to about 98% of a water-based solvent system (carrier medium), PA1 (b) from about 1% to about 20% of one or more colorants, and PA1 (c) from about 0.1% to about 20% of carboxylated lignin.
Inks are known which possess one or more of the above listed properties. However, it is difficult to "design" an ink that possesses all of the above listed properties. Often, the inclusion of an ink component meant to satisfy one of the above requirements could prevent another requirement from being met. For example, the inclusion of a polymer in the ink composition can improve the water resistance of the ink on a substrate after drying. However, the polymer can cause a reduction in the print quality and optical density, as well as impair jetting properties and long-term storage stability. Thus, most commercial ink jet inks represent a compromise in an attempt to achieve at least an adequate response in meeting all of the above listed requirements.
The use of lignin binders in ink jet inks has been disclosed. Examples include U.S. Pat. Nos. 5,702,510 and 5,281,261 (the disclosures of which are hereby incorporated by reference), which teach the use of unmodified lignin in conjunction with other polymers.
Lignin is an amorphous, high-molecular weight polymer occurring naturally in most plant fibers. The primary commercial source of lignin is wood, particularly the wood used to make paper. The major types of lignin are kraft lignin, sulfite lignin, and orgnosolv lignin, each the by-product of a distinctive pulping process. Depending upon the conditions upon which lignin is precipitated, the precipitated lignin may be either in the form of free acid lignin or a lignin salt. If lignin is precipitated at a high pH, such as about 9.5 to 10, the lignin is obtained in the form of a salt. If this lignin is further processed by washing, acidifying to a low pH, such as about 2 to 5, and further washed so as to be substantially free of salt and ash-forming ingredients, free acid lignin, known as "A" lignin, is obtained. A monovalent salt of lignin, such as an alkali metal salt or an ammonium salt, is soluble in water, whereas free acid lignin and polyvalent metal salts of lignin are insoluble in water.
It is known to employ certain amine salts of lignins as binders in aqueous printing ink compositions, as shown in commonly assigned U.S. Pat. No. 4,891,070.
It is also known to use certain lignins as binding agents for water-based ink systems incorporating pigments and/or dispersed dyes. U.S. Pat. No. 2,525,433 discloses the use of lignin dissolved in water-miscible solvent as a binding material for a pigment in printing ink. The method of U.S. Pat. No. 2,525,433 requires the use of a solubilizing agent, generally an organic amine or alcohol, to completely dissolve the lignin particles. The resultant solution when used with pigments for ink application is limited to newsprint or other porous paper. There is no film-forming action of the lignin, and no solid particles remain after the lignin is solubilized. Adhesion to coated paper, calendered paper, and sized papers is poor with excessive rub-off resulting.
U.S. Pat. No. 4,957,557 (the disclosure of which is hereby incorporated by reference) teaches the use of this submicron or small particle lignin to produce carbon black ink compositions. Inks prepared with this type of lignin, however, are not storage-stable (particularly at elevated temperatures). Over time, viscosity increases and sediments are formed, or gel-like inks are obtained. This can reduce or eliminate the amount of ink jetted from an ink jet printer and can foul the heating element of thermal ink jet printers. The method of U.S. Pat. No. 5,192,361 (the disclosure of which is hereby incorporated by reference) teaches an improvement in storage stability obtained by replacing 10-25% of the lignin with either acrylic resin, rosin resins, styrene-maleic anhydride copolymer resin, or a combination thereof. While improving storage stability, this method introduces several negative attributes with respect to its practice in ink jet ink printing. First, migration of the acrylic resin, rosin resin, or stryene-maleic anhydride copolymer resin can occur independent of the lignin when the ink in which they are contained is printed on porous substrates such as uncoated cellulosic papers. This negatively impacts the quality of the printed image. Second, the inclusion of acrylic resins, rosin resins, or styrene-maleic anhydride copolymer resins in place of lignin reduces the performance enhancing properties of the lignin in an ink jet ink. That is, the degree to which the lignin improves optical density, print quality, and drying speed decreases.
Accordingly, an object of the present invention is to provide an improved ink composition comprising a modified lignin by-product of a kraft pulping process capable of satisfying simultaneously the properties required of an ink composition for ink jet printing, especially the aforementioned properties (a) to (f). Another object of the instant invention is directed to preparation of a modified lignin by-product that provides an ink composition for ink jet printing with reduced water sensitivity and improved optical density. Another advantageous feature of the invention is to provide an ink that has excellent filterability such that the ink can be filtered without ruining the filters upon its formulation. Other objects and advantages of the present invention will become apparent from the following disclosure.